1. Field of the Invention
This invention relates to a method for preventing liquefaction of ground due to violent earthquake and the construction of a pipe for supplying and exhausting air used for the said method. The invention is applicable to very extensive fields of utility. Among those fields of utility to which the invention is particularly applicable is the prevention of severe liquefaction of alluvial ground or of reclaimed land caused by violent earthquake which combined with violent tremor causes such a destructive damage as uneven subsidence of ground surface in large scale, collapse of buildings and/or fall down of bridges, etc.
2. Description of the Prior Art
Liquefaction of ground is a peculiar phenomenon which occurs when a relatively loose sandy ground with feeble cohesion or a cohesionless ground saturated with ground water is shaken violently by earthquake.
This phenomenon can be observed when a volume of loosely packed dry sand in a container decreases when the container is shaken violently because the pore volume of the sand decreases by the shaking down motion.
Likewise phenomenon takes place when a ground of dry loose sand is shaken violently by earthquake, the ground settles down because the pore volume of the sand decreases. In the case when a ground of dry loose sand is shaken, any severe damage may not be caused by it, even though appreciable settlement of ground surface may take place.
However, in the case when the ground of relatively loose sand is saturated with ground water, the tendency of decrease in pore volume due to shaking motion causes a sudden rise of excessively high pore water pressure and contact between sand grains are lost to create a state as though sand grains drift in the ground water. This peculiar phenomenon is called liquefaction. When liquefaction of ground occurs, any obstacle in the ground lighter in unit weight than the ground floats up and anything heavier in unit weight than the ground sinks down. The liquefied ground flows slowly toward a low side even on a slope of very slight gradient which is nearly horizontal. Thus a liquefied ground loses its bearing capacity and the phenomena of pressurized sand blowing up out of the ground takes place leaving small craters at random spots.
The aforementioned liquefaction was presumed to be likely to occur in a sand of relatively uniform grain size up to 2.00 mm in 50% grain diameter D50, the relative density of which is lower than 70%.
However, in Hanshin Awaji Great Earthquake of 1995, liquefaction occurred in the ground of sand and gravel where D50 is much larger than 2.0 mm and grain size are not uniform.
The prior countermeasures for preventing the aforementioned liquefaction are, (1) methods to improve the ground so that liquefaction does not occur even though it is shaken by violent earthquake, (2) methods to design a structure so that it is not damaged fatally even when liquefaction of the ground occurs.
Among the aforementioned countermeasures (1) by improving the property of ground is a method to increase the density of ground, a method to solidify ground, a method to replace the ground with better soil, and a method to lower the degree of saturation of ground water.
The prior method proposed for lowering the degree of saturation of ground water is further divided into the methods to lower ground water level by means of deep wells or by means of drainage tunnels. However, both of those methods involve difficult problems.
By the deep-well method, the ground water is pumped out for lowering the ground water level. This method involves problem of land subsidence due to the consolidation of soft strata caused by the lowering of ground water level and its application to built-up urban areas is impracticable.
By the drainage tunnel method, the ground water is collected into porous tunnels installed at deep depth of ground and the collected water is pumped out for lowering the ground water level. This method also involves the problem of land subsidence due to the consolidation of soft strata caused by the lowering of ground water level and its application to built-up urban areas is impracticable.
As mentioned before, the prior methods proposed to prevent liquefaction by lowering the degree of saturation of ground water involve very difficult problem of land subsidence and their application to built-up urban areas is impracticable.